1. Field of the Invention
The present invention relates to devices for heating and thermal regulation of any body but more particularly of enclosures containing devices having operating characteristics which are very sensitive to temperature variations such as piezoelectric crystal oscillators.
2. Description of the Prior Art
A device for thermal regulation of an enclosure has already been disclosed in European patent No. 4,233 granted to the present Applicant. This device comprises a circuit for measuring the temperature of the enclosure and a circuit for heating said enclosure. The measuring circuit consists of a resistor bridge, one arm of which is provided with at least one heat-sensitive element, as well as an amplifier for delivering at its output a control signal which is representative of the difference between the measured temperature of the enclosure and a reference temperature, the absolute value of said control signal being adjusted by means of a negative feedback loop. The heating circuit is supplied from a voltage source and regulated by the temperature-measuring circuit. Said heating circuit is usually constituted by at least one voltage-stabilizing semiconductor circuit, a heating current being passed through said circuit and adjustable by means of a control transistor which is subjected to said control signal. In certain instances, the heating circuit can be constituted solely by the control transistor. In these two instances, the negative feedback loop of the output amplifier of the temperature-measuring circuit is taken directly from the output of said amplifier. The result thereby achieved is that the gain of the thermal regulation device must be adjusted for each product by reason of the dispersion of characteristics of the control transistor which is subjected to the control signal delivered by the output amplifier. A non-linear heating current which is a function of the control voltage is also observed.